The present invention relates to a process for oil recovery by tenside flooding.
In the extraction of oil from petroleum deposits, only a fraction of the originally existing oil can usually be recovered by primary extraction methods. In these methods, the oil is brought to the surface using the natural reservoir pressure. In secondary oil recovery, water is forced into one or several injection wells of the formation and the oil is forced to one or several production wells and thereafter brought to the surface. This so-called water flooding is a relatively inexpensive secondary measure and, accordingly, is utilized frequently. However, in many cases it results in only a minor additional oil extraction from the deposit.
An effective displacement of the oil, which, though more expensive, is urgently required from the viewpoint of the national and international economy in view of the present petroleum scarcity, is accomplished by tertiary measures. These include processes wherein either the viscosity of the oil is reduced and/or the viscosity of the flooding water is increased and/or the surface tension between water and oil is decreased.
Most of these processes can be classified as solution flooding, mixture flooding, thermal oil recovery methods, tenside or polymer flooding and/or as a combination of several of the aforementioned processes.
Thermal recovery methods involve the injection of steam or hot water, or take place as an in situ combustion. Solution or mixture processes involve the injection of a gaseous and/or liquid solvent for the petroleum into the deposit.
Tenside flooding processes, depending on tenside concentration and optionally, the type of tenside and additives, can be differentiated among tenside-supported water flooding, the usual tenside flooding (low-tension flooding), micellar flooding, and emulsion flooding. All are primarily based on a strong reduction of the surface tension between oil and deposit water. However, in some instances, especially in the presence of reIatively high tenside concentrations, water-in-oil dispersions are produced. These exhibit a markedly increased viscosity as compared with the oil, so that, tenside flooding also attempts to achieve a reduction of the mobility ratio whereby, as is known, the degree of efficiency of oil displacement is raised. Genuine polymer flooding is predominantly based on attaining the last-mentioned effect of a more favorable mobility ratio between oil and flooding water.
Heretofore, organic sulfonates, such as alkyl, alkylaryl, or petroleum sulfonates, have been described above all others as the oil-mobilizing tensides. However, these compounds have a very low tolerance limit with respect to the salinity of the waters present in the deposits. Salt concentrations of as low as 1,000 ppm are considered problematic. The sensitivity of these tensides against alkaline earth metal ions is especially pronounced. In this connection, about 500 ppm is assumed to be the upper critical limit concentration (U.S. Pat. No. 4,110,228). In this case, precipitation products occur in the presence of higher salt concentrations. These can lead to plugging of the formation. However, since many deposit waters possess substantially higher salinities, e.g., in Northern Germany up to 250,000 ppm, attempts have been made to find ways to exploit the otherwise good oil-mobilizing properties of the organic sulfonates for deposit systems of higher salinity. Organic sulfonates do show a lesser electrolyte sensitivity in mixture with cosurfactants such as alcohols or nonionic tensides, but in such cases the oil-mobilizing effect is highly impaired as well.
In contrast to this group of compounds, alkyl or alkylaryl polyglycol ether sulfates or carboxymethylated alkyl or alkylaryl ethoxylates show good compatibility even with extremely high salinities (e.g. 250,000 ppm) of the deposit waters. Since the oil-mobilizing effect of these tensides is high [H. J. Neumann, "DGMK BERICHTE" [Reports of the German Society for Petroleum Technology and Carbon Chemistry], Report 164 (1978); D. Balzer and K. Kosswig, Tenside Detergents 16:256 (1979)] and their manufacture is simple and economical, this class of compounds is very highly suitable for use in oil displacement in medium and high-salinity deposit systems (30,000-250,000 ppm total salt content).
In numerous investigations on model formations using carboxymethylated ethoxylates as the tensides, however, it has been observed that the tertiary oil was transported primarily as an emulsion. Since O/W emulsions are involved, the separation of which into an oil phase and a water phase represents an expensive step in petroleum technology, this imposes a considerable disadvantage on the process.